Omni-steerable cardioid antenna

ABSTRACT

An airborne IFF transponder antenna system which produces either omnidirectional or steerable-cardioid azimuth plane patterns, by the use of a flush dual-mode coaxial-line type cavity radiator which operates in the TEM mode and in the crossed TE11 mode.

United States Patent David 1 Jan; 23, 1973 [54] OMNl-STEERABLE CARDIOID2.913.723 11/1959 Thourel ;.....343 s54 ANTENNA 3,258,774 6/1966 Kinsey..343/854 3,281,843 10/1966 Plummer ..343/ss4 [75] I e Si y im h3,308,469 3/1967 Dl'abOWiIClL. .34s/s54x 3.441.931 41969 0 b .343 789X[73] Assignee: The United States f America as 3/1972 13:361. .343/854Xrepresented by the Secretary of the Navy Filed: Aug. 5, 1971 Appl. No.:169,284

U.S. c1. ..343/797, 343/789, 343/854,

343/873 Int. Cl. ..H0lq 21/24 Field of Search ..343/797, 789, 854, 754

Primary Examiner-John S. Heyman Attorney-41. S. Sciascia and P.Schneider [57] 7 ABSTRACT An airborne [FF/transponder antenna systemwhich produces either omnidirectional or steerable-cardioid azimuthplane patterns, by the use of a flush dualmode coaxial-line type cavityradiator which operates in the TEM mode and in the crossed TE mode.

12 Claims, 11 Drawing Figures PATENIEDJIII23 (97s 3.713.167

sum 2 0F 2- cROssEO TE MODES 3 0- I80 VARIABLE -(0 db COUPLER 4 4 I ICOUPLER 44 4/ FIG. 5 /0 I (76. 6 /0\ ELEVATION ANGLE (DEGREES) AZANGLE=O ELEVATION ANGLE (DEGREES) AZ ANGLE=O I.o .5 I O .s LO LO .5 o .5L0

RELATIVE AMPLITUDE I RELATIVE AMPLITUDE (VOLTAGE RATIO I.O5+(do1)(VOLTAGE RATIO I.OE+IdbI) F /6. 7a FIG. 8a

AZIMUTH ANGLE AzII'AuTH ANGLE (DEGREES) EL ANGLE=O (DEGREES) 2oe ELANGLE= O 270' 27o. RELATIVE AMPLITUDE RELATIVE AMPLIIUDE VOLTAGE RATIOIO- -'-adbl) (VOLTAGE RATIO |.0:-5 do!) OMNI-STEERABLE CARDIOII) ANTENNABACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates generally to improvements in an IFF transponder andmore particularly to a new and improved antenna system which produceseither omnidirectional I or steerable-cardioid azimuth plane patterns.

2. Description of the Prior Art Steerable antenna patterns have beenachieved by many means in the prior art. One such means is an over sizedwave guide having four cavities mounted about the waveguide and coupledthereto. The signal is split into higher order modes by the tunedcavities. Movement of the radiation phase center, from the center of theantenna aperture, by tuning the four cavities in sequence to thefrequency of the signal achieves a conical scan. Others have usedfeedback slot antennas to achieve a TEM single frequency scanning orsteerable beam. An array of log-periodic antennas have been used toachieve omni-directional and steerable antenna patterns. This has beenachieved by using a common signal source and a manual adjustment of theinductors and capacitors in each of. the feeds of the antennas in thearray. All the systems in the prior art have achieved steerability bymechanical or electrical manipulation of a single mode signal.

SUMMARY OF THE INVENTION OBJECTS OF THE INVENTION An object of theinvention is to provide an improved feed system for an antenna.

Another object of the invention is to provide a new and improved antennastructure that may be suitably integrated with the outer surface of thevehicle.

A further object of the invention is to provide a microwave radiatorwhich is small in size and employs annular apertures.

A still further object of the invention is to provide a fixed antennawhich permits, in the horizontal plane,

' either a omnidirectional radiation pattern or a unidirectionalradiation pattern electronically steerable at any azimuth direction,over wide frequency band.

A still further object of the invention is to provide a radiator capableof transmitting in the TEM mode and the crossed TE mode.

Other objects, advantages, and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram inaccordance with the invention;

FIG. 2 is a diagram showing a preferred embodiment of the radiatorcircuit;

FIG. 3 is the side view of the coaxial-line radiator;

FIGS. 4A and B are the top and bottom view, respectively, of the printedcircuit board of FIG. 3;

FIGS. 5 and 6 are circuit diagrams of the RF control circuit in theomnidirectional and unidirectional modes, respectively;

FIGS. 7A and B show typical radiation patterns for antenna constructedin accordance with the present invention, in the omnidirectional patternmode;

FIG. 8, A and B, show typical radiation patterns for antenna constructedin accordance with the present invention, in the steerable-cardioidpattern mode.

cavity DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1,there is illustrated in block diagram form, the antenna system accordingto the present invention. The RF input/output port 10 is connected to RFcontrol circuit 11. Two conductors, l2 and 13, feed the TEM mode signaland the crossed TE mode signals, respectively, to the radiator circuit14. The signal from the radiator circuit 14 is applied to the four ports15, 16, 17 and 18 of radiator 19. The type of transmission or receptionis determined in the RF control circuit by combinations of the signalson lines 12 and 13, as applied through the radiator circuit 14 to theports l5, 16, 17 and 18 of the radiator 19.

FIG. 2 is a detailed electrical schematic of radiator circuit 14. TheTEM mode input over line 12 is fed to the sum input of hybrid junction23. The outputs of hybrid junction 23 are fed to the sum inputs ofhybrid junctions 21 and 22. The crossed TE mode signal is fed over line13 to a 3 db directional coupler 20. The outputs of the directionalcoupler 20 are fed to the difference inputs of hybrid junction 21 and22. The outputs of hybrid junction 21 are connected to input ports 15and 16 of the coaxial cavity radiator 19, and the outputs of hybridjunction 22 are connected to input ports 17 and 18 of coaxial cavityradiator 19.

For the omnidirectional pattern mode, the radiator 19 is excited by fourequi-phased signals at its four input ports 15 through 18. The TEM modesignal is fed over line 12 into the three hybrid junctions 21, 22 and23, which generate the'equi-phased signals.

For the steerable-cardioid pattern mode, both the TEM mode signal overline 12 and the crossed TE mode signals over line 13 are fed through thethree hybrid junctions 21, 2'2 and 23 and through directional FIG. 3 isa detailed drawing of the coaxial cavity radiator 19. The cavity whichis made up of outer conductor 30 and inner conductor 31 and is filledwith low dielectric constant foam 32. The top of the cavity is closed bya teflon glass window 33. A toroidal printed circuit board 36 isattached in the coaxial cavity and supported by the foam. RF input 34 isconnected by, coaxial line 35 to the printed circuit board 36. FIG. 4Bshows printed circuit 36 with the four equally spaced coaxial lineconnectors 35 connected to one-half of a dipole exciter 38. The otherhalves 37 of the dipole exciters are shown in the top view of 4A ofprinted circuit board 36.

In designing a radiator assembly which would give optimum performancewith a omnidirectional pattern and a cardioid unidirectional pattern,optimum omnipattem performance was defined as a shaped elevation patternwhich provides increased horizon gain at the expense of reducing gain innear zenith directions. Large diameter slots (greater than one wavelength) radiate most of their power in near zenith (broadside)directions. Small diameter concentric annular slots about one-half wavelength diameter or less would be needed to realize optimum design.

Optimum cardioid performance is here defined to be a cardioid-azimuthpattern with minimum signals response (pattern notch) over a large rangeof elevation angles. It should be noted to obtain large elevation extentof the angular notch, requires the use of multiple concentric rings. Theouter rings must be large in size and, consequently, they reduce horizongain in the direction of the cardioid maximum. The optimum design for aradiator which produces both modes has been found to consist of a 0.9wave length diameter aperture backed by about a 0.2 wave length deepcoaxial line cavity. This aperture is equivalent to a one-half wavelength diameter annual ring radiator. The cavity is excited by fourdipoles symmetrically located every 90 in circumference on a printedcircuit board, as shown in FIGS. 4 A and B. It should be noted thatonehalf of each dipole is printed on each side of a printed circuitboard 36 to facilitate the connection of the coaxial line 35.

The impedance radiation frequency for the radiator operating either inthe TEM or the cross TE modes is equivalent to that of the double-tunedresonant circuit. The dipoles are the primary series-tuned resonators.The aperture and the cavity form the secondary parallei-tunedresonators. Coupling between resonators is determined by the location ofthe dipoles with respect to the aperture. The displacement from theaperture in effect determines the mutual inductive coupling between theprimary and secondary resonators. The equivalent circuit parametervalues for both modes, are quite different, thus the radiator designmust be a compromise impedance match for both modes.

The average resonant frequency of both TEM and TE 'modes has been foundto be 1045 Mhz, which is very close to the design center frequency of1060 Mhz. This scheme of having the average resonant frequency equal tothe design center frequency and having the actual TEM and TE resonancesdetuned high and low, respectively, by about 7 percent result in thebest compromise radiator design.

As described earlier, the two cross TE, modes are excited in phasequadrature with a 3 db directional coupler 20. The reflection at theinputs to the coupler is less than that at the individual TE mode ports,because most of the reflected power is dissipated in the terminated portof the coupler. The measured standing wave ratio at the coupler inputport is less than 3 db over a band from 900 to 1300 Mhz.

The details of the RF control circuit 11 is shown in FIGS. 5 and 6,which are in the omnidirectional and the steerable-cardioid patternmodes, respectively. The components of the RF control circuit 11 are anRF input/output port 10, two transfer switches 40 and 50, a negative 10db directional coupler 60, a or 180 step phase shifter 74 and 75,respectively, a 0 180 continuously variable phase shifter 84, crossed TEmode signal line 13, and TEM mode signal line 12. The 0 or I80 stepphase shifter 74 and 75 is constructed of two single-pole double-throwswitches 70 and 80, and two cables 74 and 75 that differ by l80 inelectrical length at 1060 Mhz.

In the omnidirectional mode of FIG. 5, the input signal through port 10is fed into transfer switch 40 through terminal 41 and out of terminal42 to terminal 52 of transfer switch 50. The output of terminal 53 is aTEM mode signal. In FIG. there is no signal from port to the crossed TEmode line 13, because of the 7 position of transfer switch 40. In thesteerable-cardioid mode, as shown in FIG. 6, input signal from port 10enters transfer switch 40 through terminal 41 exits through terminal 44to directional coupler 60. The signal from directional coupler 60 exitsthrough terminal 63 to transfer switch 50 to the TEM mode signal line12. There is also a signal from terminal 64 of coupler 60 through switchinto either the 0 or 180 step phase. shifter, 74 and 75, respectively,through switch to the continuously variably phase shifter 84 tothe crossTE mode line 13. It should be noted that the switching from'theomnidirectional mode to the steerable-cardioid mode is achieved byturning the contacts of transfer switchs, 40 and 50, through each.

Steerability of the cardioid null over 360 in the azimuth direction isachieved by varying phase shifter 84 from 0' to 180 with switches 70 and80 connected to the step phase shifter 74 for the first 180 and thenswitching switches 70 and 80 to the 180 step phase shifter 75.

It has been observed that essentially all the signals introduced at port10 will be channeled to TEM mode line 12. This circuit path has constantloss and linear phase with frequency over the 900 to I300 Mhz frequencyband. In the steerable cardioid mode the insertion losses of the circuitpath to lines 12 and 13 differ by about I 1 db. This amplitudedifference places the elevation angular location of the radiation powerminimum on the horizon. The RF control circuit was designed with therelative transmission phases, to the two lines 12 and 13, linear withfrequency and with approximately the same slope, in order to preventwandering of the azimuth cardioid minimum direction with 7 frequency.

FIG. 7 shows the omnidirectional radiation pattern of the antenna systemat I090 Mhz or in the TEM mode. It may be observed that the antenna doesindeed provide nearly identical gain in all azimuth directions;

that is, it provides omnidirectional coverage. The

Typical-steerable cardioid patterns of the antenna system operating atl090Mhz is shown in H0. 8. Again,' it may be observed that theazmuth-plane patterns of the antenna are indeed cardioid shaped. Thecardioid patterns may be'steered by the RF control circuit so that theminima points to any azimuth direction. it is interesting to note thatthe average 3 db bandwidth of the cardioid maximum is approximatelyequal to l70; this compares with the theoretical band-width of 180 foran ideal cardioid-radiation pattern. The elevation-plane patternpresented is for the azimuth direction (0-l 80), which contain thecardioid minima. Elevation-plane patterns for other azimuth directionsare different.

The above disclosed invention is an airborne lFF transponder antennasystem which can provide either omnidirectional or steerable cardioidazimuth plane patterns. The particular mode of antenna pattern operationis to be selected by manual control switch located within the aircraft.The omnidirectional pattern is to provide adequate communications signalstrength for both air-to-surface and co-altitude air-to-air performance.The steerable cardioid mode of the antenna is to provide a means fordecreasing the intensity of both transmitted and received signals in aselective direction.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefor to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. An antenna system comprising:

coaxial-line cavity radiator means, the cavity being tuned to resonatein two modes;

radiator circuit means connected to the radiator means for providingsaid radiator means with excitation signals;

RF control means connected to the radiator circuit means and providing aplurality of signals,

said RF control means comprising mode selection means for determiningeither an onmidirectional pattern or an electronically steerableunidirectional pattern, and phase-shifting means for determining thedirection of the steerable unidirectional signal,

said RF control means having an input port.

2. The antenna system according to claim 1 wherein:

said radiator means is tuned to the TEM mode and the two crossed TE,modes;

said TEM mode is used alone for omnidirection patterns; and

a combination of TEM mode and crossed TE modes are used for thesteerable unidirectional pattern.

3. The antenna system according to claim 1 wherein said radiator circuitmeans comprises two input ports, three hybrid junction means and adirectional coupler means all of which are interconnected to receive theplurality of signals from the RF control means and provide theexcitation to said radiator.

4. The antenna system according to claim 3, wherein:

said hybrid junction means produce equiphase signals for theomnidirection mode; and

said hybrid junction means and said directional coupler means providethe signals for the steerable unidirectional mode.

5. The antenna system according to claim I wherein:

said mode selection means comprises two transfer switches and adirectional coupler;

said phase shifting meanscomprises a 0 continuously variable phaseshifter connected by a switching means to a 0 or 180 step phase shifter.

6. The antenna system according to claim 5 wherein:

said RF control circuit has two output ports;

and said mode selection means and phase shifting means areinter-connected to produce a. one of the radiators modes on one of theoutput ports for the omnidirectional pattern and b. simultaneously, bothof the radiators modes on each of the output ports separately for thesteerable unidirectional pattern.

7. The antenna system according to claim 6 wherein:

said radiator means is tuned to the TEM mode and the two crossed TEmodes;

said TEM modes is used alone for omnidirection patterns; and

a combination of TEM mode and crossed TE modes are used for thesteerable unidirectional patterns.

8. The antenna system according to claim 1 .wherein:

said radiator. means is a coaxial-line cavity radiator comprising foursymmetrically located dipoles.

9. DuaLmode antenna means for radiating an omnidirectional orsteerable-cardiord pattern, as desired, comprising:

radiating means containing a cavity therein and including at least fourcoaxial-line dipole antenna means located symmetrically around saidcavity, I

said cavity being excited to radiate electromagnetic energy when saiddipole means are energized; and 7 means for feeding signals of two typesto said dipole antenna means, the first of which types excites saidcavity to radiate an omnidirectional radiation pattern and the second ofwhich types excites said cavity to radiate a steerable cardiord pattern.

10. An antenna as in claim 9, wherein the first type of signal fed tosaid dipoles excites resonance of said cavity in a TEM mode and thesecond type of signal fed to said dipoles excites resonance of saidcavity in a TEM mode and in a pair of orthogonally related TE modes.

11. An antenna as in claim 9, wherein the diameter of the aperture isapproximately 0.9 of a wavelength at the center design frequency and thedepth of the coaxial-line cavity is approximately 0.2 of a wavelength atthe center design frequency.

12. An antenna as in claim 11, wherein said feeding means includes rfcontrol circuit means and radiator circuit means, said radiator circuitmeans including three hybrid junctions, each with a sum (2) and adifference (A) input port and each with two output ports, and a 3 dbcoupler having an input port and at least two output ports,

said TEM signal being fed to the 2 port of the first hybrid junction,the output ports of which feed the 2 ports of the other two hybridjunctions,

said TE signals being fed to the input port of said coupler, one ofwhose output ports feeds the A port of the second hybrid junction andthe other of whose output ports feedsthe A port of the third hybridjunction,

the four output ports of said second and third hybrid junctions eachbeing connected to a different one of said dipole elements.

t a: 1 r

1. An antenna system comprising: coaxial-line cavity radiator means, thecavity being tuned to resonate in two mOdes; radiator circuit meansconnected to the radiator means for providing said radiator means withexcitation signals; RF control means connected to the radiator circuitmeans and providing a plurality of signals, said RF control meanscomprising mode selection means for determining either anonmidirectional pattern or an electronically steerable unidirectionalpattern, and phaseshifting means for determining the direction of thesteerable unidirectional signal, said RF control means having an inputport.
 2. The antenna system according to claim 1 wherein: said radiatormeans is tuned to the TEM mode and the two crossed TE11 modes; said TEMmode is used alone for omnidirection patterns; and a combination of TEMmode and crossed TE11 modes are used for the steerable unidirectionalpattern.
 3. The antenna system according to claim 1 wherein saidradiator circuit means comprises two input ports, three hybrid junctionmeans and a directional coupler means all of which are interconnected toreceive the plurality of signals from the RF control means and providethe excitation to said radiator.
 4. The antenna system according toclaim 3, wherein: said hybrid junction means produce equiphase signalsfor the omnidirection mode; and said hybrid junction means and saiddirectional coupler means provide the signals for the steerableunidirectional mode.
 5. The antenna system according to claim 1 wherein:said mode selection means comprises two transfer switches and adirectional coupler; said phase shifting means comprises a 0 - 180*continuously variable phase shifter connected by a switching means to a0* or 180* step phase shifter.
 6. The antenna system according to claim5 wherein: said RF control circuit has two output ports; and said modeselection means and phase shifting means are inter-connected to producea. one of the radiator''s modes on one of the output ports for theomnidirectional pattern and b. simultaneously, both of the radiator''smodes on each of the output ports separately for the steerableunidirectional pattern.
 7. The antenna system according to claim 6wherein: said radiator means is tuned to the TEM mode and the twocrossed TE11 modes; said TEM modes is used alone for omnidirectionpatterns; and a combination of TEM mode and crossed TE11 modes are usedfor the steerable unidirectional patterns.
 8. The antenna systemaccording to claim 1 wherein: said radiator means is a coaxial-linecavity radiator comprising four symmetrically located dipoles. 9.Dual-mode antenna means for radiating an omnidirectional orsteerable-cardiord pattern, as desired, comprising: radiating meanscontaining a cavity therein and including at least four coaxial-linedipole antenna means located symmetrically around said cavity, saidcavity being excited to radiate electromagnetic energy when said dipolemeans are energized; and means for feeding signals of two types to saiddipole antenna means, the first of which types excites said cavity toradiate an omnidirectional radiation pattern and the second of whichtypes excites said cavity to radiate a steerable cardiord pattern. 10.An antenna as in claim 9, wherein the first type of signal fed to saiddipoles excites resonance of said cavity in a TEM mode and the secondtype of signal fed to said dipoles excites resonance of said cavity in aTEM mode and in a pair of orthogonally related TE11 modes.
 11. Anantenna as in claim 9, wherein the diameter of the aperture isapproximately 0.9 of a wavelength at the center design frequency and thedepth of the coaxial-line cavity is approximately 0.2 of a wavelength atthe center design frequency.
 12. An antenna as in claim 11, wherein saidfeeding means includes rf control circuit means and radiator circuitmeans, said radiator cIrcuit means including three hybrid junctions,each with a sum ( Sigma ) and a difference ( Delta ) input port and eachwith two output ports, and a 3 db coupler having an input port and atleast two output ports, said TEM signal being fed to the Sigma port ofthe first hybrid junction, the output ports of which feed the Sigmaports of the other two hybrid junctions, said TE11 signals being fed tothe input port of said coupler, one of whose output ports feeds theDelta port of the second hybrid junction and the other of whose outputports feeds the Delta port of the third hybrid junction, the four outputports of said second and third hybrid junctions each being connected toa different one of said dipole elements.